Electric discharge lamps



May 23, 1961 w. J. scoTT 2,985,787

ELECTRIC DISCHARGE LAMPS Filed Nov. 17, 1959 3 Sheets-Sheet l //v I/f/VTOR W/LL/HM JOSE 56077 1 7 T TOR/v5 y 1961 i w. J. SCOTT 2,985,787

ELECTRIC DISCHARGE LAMPS Filed Nov. 17, 1959 3 Sheets-Sheet 2 W/LL M J05 0T7- QTTOFP/VEY May 23, 1961 w. J. sco'r'r ELECTRIC DISCHARGE LAMPS 3 Sheets-Sheet 3 Filed Nov. 1'7, 1959 F|G.7b.

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/N VEN TOR M v HTTOFH/VEY' United States Patent 2,985,787 ELECTRIC DICHARGE LAMPS William Joseph Scott, Rugby, England, assignor to The British Thomson-Houston Company Limited, London, England, a company of Great Britain Filed Nov. 17, 1959, Ser. No. 853,503

Claims priority, application Great Britain Nov. 19, 1958 12 Claims. (Cl. 313220 This invention relates to electric discharge lamps and is particularly, although not exclusively, concerned with high and low pressure metal vapour discharge lamps, including fluorescent lamps.

The envelopes of electric discharge lamps are commonly made from glass or fused silica and if the lamp is operated in conditions in which a pressure materially higher than atmospheric is present in the envelope during operation and it is essential for the surroundings to be protected against fracture of the envelope, it has hitherto been customary to "provide a protective metal and glass lantern or casing around the envelope. There is also an upper limitation to the size of conventional low pressure lamps on account of the risk-of fracture of glass.

The main object of the present invention is to provide a 'constructure of envelope which is inherently stronger than envelopes of the'customary type and is substantially explosion and implosion proof.

A further object of the invention is to provide a discharge lamp of a construction enabling the lamp to constitute a light emitting surface of extended area.

A still further object is to provide a unitary electric discharge lamp capable of constituting a light source equivalent to a plurality of discharge lamps of conventional construction.

Another object is to provide a discharge lamp with an envelope capable in conjunction with a sinuous'arc path of constituting reactive impedance'for limiting the arc current when the AC. supply to the lamp is of high frequency. i

An electric discharge lamp according to the invention is provided with an envelope of metal containing perforations filled with a transparent or translucent vitreous material hermetically sealed to the metal.

In order to prevent the interior portions of the metal envelope from short-circuiting the electric discharge present within the envelope, the vitreous material which seals the perforations in the metal is extended to cover such internal surfaces exposed to the atmosphere 'in which the discharge takes place, so that a continuous surface of vitreous material is presented to the discharge. Any stem or pinch extending within the envelope and supporting an electrode therein is then caused to be sufliciently continuous with the coating of vitreous material present on the inner wall of the metal envelope. Discontinuities in the coating, such as places where the metal walls are joined together by welding, are suitably shielded or placed.

remote from the discharge. 7

If the inner coating of vitreous material is porous the pores must be long enough to block oif the discharge.

The vitreous material, suitably glass, employed for sealing the perforations in the metal envelope, and, if desired, covering the internal surface thereof, is such as to be compatible with the material with which the envelope is mainly constructed, i.e. the vitreous material must wet and hermetically seal to the metal and possess a thermal expansion co-efiicient either matched to that of the metal or somewhat less than that of the material over the range extending from the melting point of the vitreous material to ambient temperature. If a glass is used and the thermal expansion co-efiicient of the glass is matched to that of the metal then, apart from external causes, no stress will be produced in the glass when, the metal having been coated with the glass at the melting point of the glass, is restored to ambient temperature. If the thermal expansion co-efficient of the glass is less than that of the metal then, neglecting external forces and edge shape effects, only compressive stress will be produced in the glass at ambient temperature. This is considered to be the more desirable condition.

Many combinations of metal and glass are known which will fully fill the considerations above expressed; for example in our prior British patent specification No. 572,- 020 a composition of glass capable of sealing to iron is given such that only compressive stress is present in the glass at ambient temperature. Such a glass may be employed in carrying out the present invention.

The invention will be better understood on considera tion of the following description of the accompanying drawings, in which:

' Fig. 1 shows partly in cross-section a discharge lamp having a metal envelope'of tubular shape constructed in accordance with the invention,

Fig. 2 shows in plan view a discharge lamp providing an extended light surface and incorporating a metal envelope with a zig-z'ag' discharge path,

Fig. 3 is an end view of Fig. 2,

Fig. 4 is an exploded view of the parts of a discharge lamp providing an extended light emitting area before the parts are sealed together,

Fig. 5 is across-section of the arrangement shown in Fig. 4 when the parts have been assembled and sealed and/ or welded together,

Fig. 6 is a perspective view partly in cross-section of a portion of a discharge lamp constituting a further embodiment of the invention,

Figs. 7a and 7b show detail views of part of the discharge lamp shown in Fig. 6,

Fig. 8 shows in cross section a portion of a discharge lamp intended for giving an extended light emitting area and constituting a modified construction of the kind illustrated in Fig. 6, and

Fig. 9 is a plan view of the-construction shown in Fig. 8. a 1

Referring to Fig. 1, a discharge lamp of the fluorescent type is shown as consisting of a tubular envelope 1 of metal having supported at each end thereof an electrode 2 of the thermionic type carried by stems 3 in the form of metal cups through which the electrode 2 is insulatingly-supported, the stems closing the ends of the envelope. The envelope is evacuated andfilled with an ionisable medium, such as argon with a trace of mercury vapour, through which a discharge takes place during operation between the electrodes 2 One or both of the electrodes is shown as mounted on a tube 4 which is used for effecting the evacuation and gas-filling of the envelope, the tube being indicated as sealed-oh at 5 by pressing the side Walls at the end of the tube together in orderto forma vacuum-tight seal.

The envelope 1 is provided with a pluralty of spaced perforations or apertures 6 of any suitable shape for the emission of visible light'obtainedas aresult of the discharge between the electrodes 2; the perforations .6 are filled with a -light transmissive vitreous material .7 hermetically sealed to the metal envelope so as to cause the latter to forrna complete envelope capable of evacuation and gas-filling. The vitreous material 7 is convenient 1y of glass and in order to ensure that the discharge not short-circuited by the presence of the metal envelope in its vicinity, the glass 7 filling the perforations 6 is extended, as indicated at 8, to cover the whole of the interior of the metal portion of the envelope which is preferably treated so as to be a good reflector of light as will be described later. If the discharge within the envelope is largely of an ultra-violet emitting nature, such as the low pressure are obtained by a discharge in mercury vapour, the interior wall of the envelope is of, or is coated with, a phosphor 12a which fluoresces under the action of the UV. radiation emitted by the discharge. If desired, the coating glass may itself be fluorescent. The diffused glow thus produced by the excitation of the phosphor is emitted through the perforations 6. By virtue of the use of metal for the major portion of the envelope of the lamp the lamp may be rendered substantially proof against fracture, either by explosion or implosion.

The seals at the end of the envelope through which the electrodes 2 are supported are shown as consisting of metal stems 9 sealed at their outer ends to the wall of the envelope at 10, e.g. by welding. Stems 9 are formed with a flanged inner end 11 having a central hole for the passage of the tube 4 supporting the electrode 2, the tube being sealed in the aperture by a glass-to-metal seal 12.

The stem 9 is shown as being welded to the end of the tubular envelope at however the seal between the stem to the tube 1 may be provided by the layer of glass which is formed on the outside of the stem 3 and on the inside of the tube 1.

It is important that the glass used for filling the perforations 6 and coating the inner wall of the metal tube 1 be compatible with the metal of which the envelope is mainly constructed. The tube 1 may thus be made of a nickelcobalt-iron alloy of well-known type, to which a glass of hero-silicate composition will readily seal; alternatively, the metal parts of the tube may be of iron or steel to which the composition of glass mentioned in British patent specification No. 572,020, above quoted, can be sealed.

The glazing of the metal tube 1 forming the major portion of the envelope can be efiected by dipping the preformed metal into molten glass having a suitable composition and on withdrawal from the molten glass allowing the excess glass to drain from the metal. Alternatively, vitreous enamelling or other techniques may be employed for the coating. The envelope itself may be pierced from tube or fabricated from sheets, strips or rods, joined as by welding.

The construction of electric discharge lamp in accordance with the invention makes possible fluorescent lamps of large size possessing an extended discharge space of a zig-zag, a helical or other character. The arrangement illustrated in Figs. 2 and 3 represent a fluorescent discharge lamp of this character.

The arrangement shown in Fig. 2 provides a zig-zag path for the discharge between an electrode 13 and an electrode 14, by means of transverse partitions 15 in a metal box 16 having plates 17 for completing the enclosure of the discharge, the cover plates being welded to the sides of the box and to the partitions before glazing. The partitions in common with the other walls are insulated and are put there in order to compel the arc to take the tortuous path indicated by the chain-dotted lines. One, or both, cover plates 17 of the box is formed with apertures 18 of any suitable size, shape and arrangement, which are filled with glass in a manner similar to the perforations 6 in the arrangement shown in Fig. 1. The electrodes 13, 14 are sealed into tubular projections, or ports, 19 formed in the end walls of the tray by the cup-shaped seal similar to those provided in the arrangement shown in Fig. 1. The tubular projections 19 are also the access ports provided for coating the interior with the glass enamel reflector and the phosphor coatings. The ends of the box slope towards the ports 19 to facilii tate coating by conventional methods. Auxiliary starting electrodes (not shown) may be fitted through the intermediate ports 19.

In order to assist in the reflection of light from the internal metal ports, the latter may be provided, prior to the coating of the inner surface with the glass, with a coating which serves as an eflicient reflector for the radiation emitted by the discharge. For this purpose the metal may be coated on its inner surface with rhodium prior to the provision of the coating of glass on at least the inner surface thereof. Alternatively, a coating of white vitreous enamel or other insulating and reflecting layer may be applied to the box. A coating of magnesium oxide, titanium oxide or other reflecting insulative material may be applied to the glass layer after it has been applied to the metal surfaces. As a rule the cover plate(s) 18 will be sealed to the sides by welding, but in certain designs the glass coating may be used as a sealing material.

A further embodiment of the invention is illustrated in Figs. 4 and 5, in which the envelope of the lamp is in the form of a flanged metal tray 20 to which an apertured cover member 21 is secured. The apertures in the cover member 21 are shown at 22 as being of rectangular form and are sealed with glass in the manner explained hereinbefore. The discharge occurs between electrodes 23, 24 sealed through the end wall of the tray by metal-glass seals 25, the discharge being caused to take a circuitous path between the electrodes 23, 24 by means of a central barrier member 26 formed integrally with the tray. The barrier member 26 may terminate at its inner end in a vertical wall 27 into which is attached an evacuating tubulation 29, also of metal. The end of the tubulation 29 is sealed off in the manner proposed in connection with the tube 4 shown in Fig. l by closing the end of the tube after evacuation and gas-filling of the envelope. In order to enable the cover member 21 to be sealed to the tray 20 the edges of the cover member and tray are formed with rims 30 which are welded together as indicated at 31. The rims 30 are indicated as being slightly flanged together so as to leave a space between the cover member and the flanged edge of the tray 20 into which the glass 32, which covers the interior faces of the cover plate and tray, can extend. The whole of the interior surface of the cover member and tray are coated with glass, preferably as a white enamel, so that the barrier member 26 is closed against the cover member by the sealing together of the coatings of glass on the cover member and tray as indicated at 33.

A layer of phosphor is applied to the interior surfaces of the tray and, if desired, to the cover member windows which together are to constitute the walls delimiting the arc path.

A modification of the construction shown in Figs. 4 and 5 is shown in Fig. 6, in which the rims of the tray 20 and cover member 21 are welded and sealed together directly or by means of a folded strip 34 of metal placed over the adjacent rims and welded thereto. In the arrangement shown in Fig. 6, the tray is made from iron sheet covered with white vitreous enamel. In this embodiment the window member is curved and ribbed and indentations 35 are made in the base of the tray 20 along the arc path in order to diminish the cross-section of the space through which the arc is to pass in such a way as to spread the arc and increase the light output and/or efficiency of the lamp as well as to strengthen the envelope. The barrier member 26 is shown in this embodiment as being separated from the cover member 21 by an insulating gasket 36 which may be formed of glass wool or other refractory material. The presence of the gasket 36 prevents short-circuiting of the are directly between the electrodes and causes the arc to take the circuitous path around the barrier member. The use of gasket 36 avoids a necessity for making a very close fit between barrier member 26 and the cover member 21, which may thus be allowed to warp a little in the processing without any harm being done. In this embodiment also the apertures in the cover member are sealed by glass panes 38 which are contained by metal reinforcement 39. Reinforcement 39 acts as support for the panes which are arranged in groups in a larger window frame.

Figs. 7a and 7b show alternative constructions in which the glass of the windows adjoins only with the vitreous enamel coating on the inner rim of the metal reinforcement 39 (Fig. 7a), and in which the window glass extends over the enamel coating the inside of the metal reinforcement (Fig. 7b).

In the arrangement shown in Figs. 8 and 9, which indicate a section of a corner of a discharge lamp of large size, the envelope of the discharge lamp is in the form of a hollow metal box of integral construction, the front wall 40 of which is constructed with a number of windows 41 of generally rectangular shape into which panes '42 of glass are sealed. The rear wall 43 of the box is formed with depressions 44 opposite each window, the function of which is both to increase the rigidity of the rear wall and also to form the cross-sectional area of the discharge space 45 in which the arc discharge occurs to a shape which gives maximum light output. The front and rear walls are also supported against one another to resist the external force applied to the box by reason of the differential pressure resulting from the partial vacuum within the box and the atmospheric pressure outside the box by means of partitions 46 which also serve to cause the arc path to be of a convoluted character. In order to assist the panes 42 to resist the atmospheric pressure to which they are subjected, they are made somewhat convex, as indicated in Fig. 8, and the rim of each aperture 41 is formed with a flange providing an outwardly turned rim 47 to which the edge of the pane 42 is sealed. As in previously described constructions the whole of the interior of the box is coated with a vitreous material, preferably in the form of a white vitreous enamel, to increase the reflection of light therefrom. This coating is indicated at 46. The coating may also extend over the outer surface of the side wall of the box, as indicated at 49, and, if desired, over the rear wall of the box. Subsequently, panes 42 are sealed against the rim of the open ings in which they are each located by means of a soft glass 50 which acts as a solder between the glass constituting the pane 42 and the vitreous enamel coating on the wall of the box. This sealing may be done subsequent to the enamel and phosphor coating operations which are thereby facilitated.

Electrodes (not shown) may be sealed through any wall of the box in any convenient position by glass-tmetal seals as hereinbefore described. Furthermore, the vitreous enamel coating on the inner wall of the box may be rendered more reflective by a light reflecting layer 51 superposed thereon. A further layer '52 of luminescent phosphor may also be provided on the inner wall of the box overlying the light reflecting layer 51, if the latter is provided. The phosphor preferably extends also over the inner surface of each of the panes 42. As previously described the box may be of iron or steel and the layer of glass in contact therewith and the panes having a composition suitable for forming a satisfactory hermetic joint with the steel.

A discharge lamp having the construction described with reference to Figs. 8 and 9 may be of large size. It is envisaged that the metal box constituting the envelope of the lamp be of rectangular form, the dimensions of which are of the order of 48" x 24" x 2%". Such a lamp may be formed with, for example, thirty-two windows in one face of the box, the windows having dimensi-ons of approximately x 5". Electrodes would then be sealed into the box at corners adjacent one edge thereof, the perforations being arranged so that the discharge takes a sinuous path extending symmetrically over the whole interior space within the box. The discharge lamp magnetic circuit providing an insulating gap, which gap 7' is present between the opposing faces of the metal sheets separated by the glass coating in certain constructions; If the discharge lamp is operated with alternating current at a high frequency the reactive impedance introduced thereby into the circuit in which the discharge path is included may be arranged to serve as part or all of the series inductance required to balance the negative voltampere characteristic of the discharge.

What I claim is:

1. An electric discharge lamp comprising a sealed envelope, spaced electrodes in said envelope, and a filling of an ionizable medium in the gaseous phase sealed within said envelope at a pressure such as to support a discharge between said electrodes when operating voltage is applied between them, said envelope consisting of metal having perforations through which light caused by said discharge emerges from said envelope, said perforations being filled with a light transmissive vitreous material hermetically sealed therein and continuous with a coating of an insulative vitreous material extending over the interior surface of said envelope.

2. An electric discharge lamp comprising a sealed envelope, spaced electrodes in said envelope, and a filling of an ionizable medium in the gaseous phase sealed within said envelope at a pressure such as to support a dischange between said electrodes when operating voltage is applied between them, said envelope consisting of metal having perforations filled with glass hermetically sealed to said metal, said glass extending as a coating over the interior surface of said envelope and being compatible with said metal.

3. An electric discharge lamp comprising a tubular metal envelope, thermionic electrodes sealed into the ends of said envelope and a filling of an ionizable medium in the gaseous phase sealed within said envelope at a pressure such as to support a discharge between said electrodes when operating voltage is applied between them, said envelope having a plurality of spaced perforations hermetically sealed with glass, and glass extending as a coating over the interior surface of said envelope.

4. An electric discharge lamp according to claim 3, in which said glass coating is fluorescent.

5. An electric discharge lamp comprising a metal envelope, spaced thermionic electrodes insulatingly sealed through the wall of said envelope, and a filling of an ionizable medium in the gaseous phase sealed within said envelope at a pressure such as to support a discharge between said electrodes when operating voltage is applied between them, said envelope being in the form of a hollow metal box, partitions extending between opposite walls of said box to define a sinuous arc path between said electrodes, openings in at least one wall of said box located between said partitions, a coating of vitreous material covering the inner wall of said box and said partitions, and a light transmissive vitreous material sealing said openings, said light transmissive vitreous material constituting a continuation of the vitreous material covering said wall.

6. An electric discharge lamp as claimed in claim 5, in which said coating of vitreous material on the metal is of a light reflecting character.

7. An electric discharge lamp as claimed in claim 5, in which a film of light reflecting material underlies said coating of vitreous material above the metal.

8. An electric discharge lamp comprising a sealed metal envelope, said envelope consisting of a shallow tray having upstanding side walls, a cover member sealed at its rim to said side walls, said cover member containing spaced apertures, partitions extending between said tray and said cover member and located between said apertures to define a sinuous arc path, a coating of vitreous material covering the inner surfaces of said tray, said partitions and said cover member, windows of a light transmissive vitreous material closing said apertures and continuous with said coating, electron-emissive electrodes insulatingly sealed through the wall of said tray at the opposite ends of said sinuous discharge path, and a filling of an ionizable medium in the gaseous phase sealed within said envelope at a pressure such as to support a discharge between said electrodes when operating voltage is applied between them.

9. An electric discharge lamp according to claim 8, in which gaskets of glass wool separate said partitions from said cover member.

10. An electric discharge lamp comprising a sealed envelope mainly of magnetisable metal, spaced electrodes insulatingly sealed to said envelope, a filling of ionizable medium in the gaseous phase sealed within said envelope at a pressure such as to support a discharge between said electrodes when operating alternating voltage at high frequency is applied between them, said envelope having perforations filled with light transmissive vitreous material hermetically sealed therein, and the arc path defined by said envelope between said electrodes being of a sinuous character.

11. An electric discharge lamp comprising a sealed envelope, spaced electrodes in said envelope, and a filling of an ionizable gaseous medium at least part of which is metal vapour sealed within said envelope at a pressure such as to support a discharge between said electrodes when operating voltage is applied between them, said envelope consisting of metal having perforations through which light caused by said discharge emerges from said envelope, said perforations being filled with a light transmissive vitreous material hermetically healed therein and continuous with a coating of an insulative vitreous material extending over the interior surface of said envelope.

12. An electric discharge lamp as claimed in claim 11, and having a coating of luminescent phosphor overlying said coating and the inner face of said light transmissive material.

References Cited in the file of this patent UNITED STATES PATENTS 2,064,354 Prouty Dec. 15, 1936 2,184,241 Barber Mar. 1 9, 1940 2,241,968 Suits May 13, 1941 2,317,265 Foerste Apr. 20, 1943 2,346,522 Gessel Apr. 11, 1944 2,405,518 Polevitzky Aug. 6, 1946 2,406,146 Holmes Aug. 20, 1946 2,457,503 Singer Dec. 28, 1948' 2,555,749 Krefit June 5, 1951 

